Off-shore pipelines such as those used to pump crude oil and natural gas ashore from off-shore drilling rigs and terminals are required to be capable of withstanding very high pressures and temperatures and are therefore typically made of metals such as iron and steel.
However, among major issues encountered with steel pipelines in general is the problem of corrosion.
In order to protect the inner bore of the pipelines from the corrosive effects of materials passing through them, such as mixtures of hydrocarbons, water and other contaminants, e.g. carbon dioxide and hydrogen sulphide, it has been already proposed to provide pipelines made of a suitable polymeric material.
Flexible pipes for conveying hydrocarbons are already well known and generally comprise, from the inside of the pipe outward, a metal carcass covered by a polymer internal sealing sheath, a pressure armour layer, tensile armour layers and a polymer external sheath to protect the entire pipe and in particular to prevent seawater from penetrating its thickness. The metal carcass and the pressure armour layer are usually made up of longitudinal elements wound with a short pitch and give the pipe its ability to withstand radial force while the tensile armour layers usually consist of metal wires wound at long pitches in order to react axial forces.
Polyvinylidene fluoride (PVDF) has been proposed as a material for manufacturing such internal sealing sheath on account of its chemical resistance to hydrocarbons at temperatures typically between 100° C. and 130° C.
Under operating temperatures and pressures, carbon dioxide and hydrogen sulphide contained in the hydrocarbons permeate through the polymer internal sealing sheath into the tensile armour layers and, in the presence of water vapour, cause corrosion of these metal reinforcements. These phenomena can impact mechanical performance of the pipe, particularly in ultra-deepwater applications.
Sour service steel grades for the pressure armour layers and the tensile armour layers are thus nowadays used.
However, such steel grades have the drawbacks of being expensive or of having poor mechanical properties, which imposes an increase in the section and the weight of the metal portions in these pipes.
Moreover, under operating temperatures and pressures, some of the gases and low-density fractions of the hydrocarbon fluids permeate through the polymer internal sealing sheath into the voids of the metal reinforcements. During decompression following production stoppage, should the pressure in the pipe reduce rapidly, these gases and low-density fractions of the hydrocarbon fluids will expand, possibly causing blistering and collapse of the internal sealing sheath.